Sunday, April 02, 2006

Wolves feasting on winter-killed bison and elk in Yellowstone

By MIKE STARK - Billings Gazette

BILLINGS (LEE) — Slabs of hardened snow and ice have kept food out of reach for some elk and bison in Yellowstone National Park this winter, leaving them to starve to death or become easy pickings for wolves. Doug Smith, head of the Yellowstone Wolf Project, said he hasn’t seen so many winter-killed elk and bison since 1996-97, the last year the park experienced severe weather that killed hundreds of ungulates.

This winter hasn’t been nearly as deadly for bison and elk as nine years ago, but it’s getting noticed by wolf researchers who track what the wolves eat. The situation at higher elevations has been driven by swings in winter conditions, Smith said. Cold snaps and snowstorms have been followed by warm periods. As snow on the ground crystallized, it formed a barrier difficult for bison and elk to penetrate as they foraged for food.

“It basically turned to concrete,” Smith said.

In those areas, weakened elk and bison have become easy meals for wolves, according to researchers conducting their annual survey of Yellowstone’s packs. Two of Yellowstone’s wolf packs, Mollie’s and Hayden Valley, have been eating nothing but bison for the last two and a half months in the park’s interior, Smith said. That’s rare because bison can be tough and dangerous prey for wolves. The difficulties for elk and bison this winter seem primarily isolated in the higher elevations.

P.J. White, Yellowstone’s lead ungulate biologist, said elk in those areas had a tough winter but those living at lower elevations fared better. Overall recruitment of elk, usually calculated as the ratio of calves to cows, seems better than the last few years, White said. The number of elk killed during the winter is up over the previous years — which saw milder weather than normal — but “it’s nothing close to 1997,” White said.

Severe weather in the winter of 1996-97 killed 300 to 400 bison and, in the northern portion of Yellowstone, more than 500 elk. Since that year, researchers usually have found about 10 elk carcasses each winter during aerial surveys around Yellowstone’s north border, according to Tom Lemke, a biologist for Montana Fish, Wildlife and Parks. Lemke, who flew over some of the elk herds on Wednesday, said that although all of the numbers have not been tallied yet, he’s not alarmed by what he has seen so far. “I would say there may be a few more (winter-killed carcasses) but I don’t think it’s anything significant,” Lemke said. Typically, the elk that linger in the high elevation tend to be the bulls, Lemke said. Wolves have a knack for finding elk in difficult environmental conditions.

This winter, Yellowstone finally received what’s considered a more “normal” amount of snow after years of mild drought. But weather patterns — especially long warm spells like the one in January — changed the texture of the snow, creating a hard crust in many places instead of the more fluffy stuff that can be easily shoved aside.

Lisa Graumlich, director of the Big Sky Institute for Science and Natural History, and others have been trying to get a better handle on recent weather and climate trends in the Yellowstone ecosystem. She’s not surprised that, despite a few cold snaps, winter this year seemed warmer. In recent years, she said, there have been “remarkably fewer” days when the temperature has dropped below zero in Bozeman when compared with records going back to 1900, and an increasing number of days when the temperature has risen above 90. “We know we’re on a trajectory where it’s just less cold,” Graumlich said.

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